Individual LEDs have been connected in series across a mains voltage (e.g., 120 VAC) so that the combined forward voltages of the LEDs are slightly less than the peak voltage. Therefore, the LEDs briefly turn on each AC cycle. The rapid blinking of the LEDs is not perceived by the human eye. Such a light source is inexpensive since no DC current regulator is needed. However, since many LEDs are used (typically 30-60), mounting and interconnecting individual LEDs on a circuit board creates a relatively large light emitter.
Serially-connected LEDs are also useful for DC applications, such as automobiles, where parallel strings of three LEDs are connected across the battery voltage (nominally 12 volts) to create any brightness level.
A series resistor is typically used to limit the current in such LED strings not using an active current source. The LEDs may be any color and may even have a phosphor coating to generate white light.
Recently, the efficiency of GaN LEDs has been increased by the removal of the transparent sapphire growth substrate, used to grow GaN LED layers. After the removal of the substrate, the exposed GaN layer is etched to thin the layer and to create a roughened surface to increase light extraction.
What is needed is an efficient technique to form series LEDs in a compact structure. What is also needed is such a compact LED structure that can reliably undergo the substrate removal process. What is also needed is such a compact LED structure that can also reliably undergo the etching process, after substrate removal, to increase light extraction.